What is an example of a trait that has variety in humans

Widow's peak, dimples, natural curly hair

Explanation:

A. increase the size of the muscle cells and the amount of blood that is delivered to the muscle

Explanation:

Muscle endurance is basically the ability of the muscles to repeat a movement many times or hold a position for a extended period of time without stopping to rest, to contract the muscles many times without tiring.

Thus if the muscle endurance has to be increased, firstly the size of cells must increase because cellular organelles specifically mitochondria , myoglobin and capillary networks in SO fibres increases during endurance exercises.  Hypertrophy is caused by resistance exercises,thus there is a high number of FG fibres in power producing muscles than in slow fibres. And for this purpose , increased blood supply to the muscle cells is also required.

Hence, If you wanted to increase your muscular endurance, it would be important to participate in activities that increase the size of muscle cells and amount of blood delivered to the muscle.